Why mockito cannot mock fun interface?

[Jacks0N23]

For ex. my code looks like this:

fun interface FunInterfaceToTest : (String) -> Int

val myInterfaceToTest: FunInterfaceToTest = spy(
    FunInterfaceToTest { mock() }
)

If new IR kotlin compiler backend is enabled then

launching test with SAM throws this:

org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class my.company.package.test$sam$my_company_package_FunInterfaceToTest$0.
Can not mock final classes with the following settings :
 - explicit serialization (e.g. withSettings().serializable())
 - extra interfaces (e.g. withSettings().extraInterfaces(...))

You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.

OR
if use anonymous object than i got this:

org.mockito.exceptions.base.MockitoException: 
Cannot mock/spy class java.lang.Integer
Mockito cannot mock/spy because :
 - Cannot mock wrapper types, String.class or Class.class

in some cases I got this:

Argument passed to verify() is of type  and is not a mock!
Make sure you place the parenthesis correctly!
See the examples of correct verifications:
    verify(mock).someMethod();
    verify(mock, times(10)).someMethod();
    verify(mock, atLeastOnce()).someMethod();
org.mockito.exceptions.misusing.NotAMockException: 

BUT
If in build.gradle file I enable old kotlin compiler backend than

android {
    kotlinOptions {
        useOldBackend = true
    }
}

everything works fine, but why??

Full Test Suite

class Test : BehaviorSpec({

  Given("test given") {
      val myInterfaceToTest: FunInterfaceToTest = spy(
          object : FunInterfaceToTest {
              override fun invoke(it: String): Int {
                  return mock()
              }
          }
      )
      When("test when") {
          myInterfaceToTest("")
          Then("so") {
              verify(myInterfaceToTest, never()).invoke(any())
          }
      }
  }
})

fun interface FunInterfaceToTest : (String) -> Int

[abelkov]

See explanation in #441